The present invention relates to a ball bearing which is to be used as it is installed in various sections to support rotating members as in hard disk drives (HDDs), video tape recorders (VTRs), digital audio tape recorders (DATs), laser beam printers (LBPs), etc. Further, the present invention relates to a method for producing a cage of such a ball bearing.
A ball bearing of the type shown in FIG. 1 is used extensively to support various kinds of rotating parts. The ball bearing provides a concentric combination of an inner race 2 having an inner raceway track 1 in the surface of an outer circumference thereof and an outer race 4 having an outer raceway track 3 in the surface of an inner circumference thereof, with a plurality of balls 5 being provided between the inner and outer raceway tracks 1 and 3 in such a way that the balls 5 can roll freely. The surfaces of the inner circumferences on both sides of the outer race 4 are engaged with the outer peripheral edges of annular seal plates 6, by which the grease contained or the dust generated in the areas where the balls 5 are in stalled is prevented from leaking to the outside of the bearing or by which the dust or dirt particles suspended in the atmosphere exterior to the bearing are prevented from entering those areas of installation.
The balls 5 are retained in a cage 7 (see FIGS. 2 and 3) in such a way that the balls 5 can roll freely. The cage 7 is formed as a unitary member by injection molding of a synthetic resin. Stated more specifically, the cage 7 provides an annular main body 8 and a plurality of retaining parts 9 that are provided on one side of the main body 8. Each retaining part 9 consists of a pair of elastic lugs 10 spaced from each other. The opposing faces of the elastic lugs 10 that constitute each retaining part 9 form concave faces of concentric spheres. Each ball 5 is pushed to install between the two elastic lugs 10 in pair as it increases the inter-lug distance, thereby allowing the balls to be held in the individual retaining parts 9 in such a way that they can roll freely.
In the case of a ball bearing of the type that promotes the lubrication of ball rolling parts by means of the grease loaded, between seal plates 6, the resistance to the stirring of the grease increases the torque necessary to rotate shafts and the like that are supported by the ball bearing and, in addition, the torque is apt to fluctuate due to the rotation. Furthermore, the grease causes another problem in that a fairly large amount of dust particles tend to be suspended in air around the ball bearing. If a grease that causes less generation of such dust is used, sound (bearing noise) is prone to develop during the rotation.
Such phenomena of large torque, torque fluctuations, extensive dust generation and production of bearing noise are prone to cause practical problems in small-diameter ball bearings with inside diameters of no more than 6 mm that are to be built in HDDs, VTRs, DATs, LBPs, etc. Particularly in HDDs, deterioration in cleanliness due to extensive dust generation will lead to head damage or reading or writing errors and, hence, should be avoided.
Under these circumstances, various improvements have heretofore been proposed. For example, Unexamined Japanese Patent Publications Nos. Sho. 61-6429 and Hei. 1-93623 teach ball bearings that obviate the use of grease by impregnating the cage with a lubricating oil, and Unexamined Japanese Patent Publication No. Sho. 64-46011 teaches a rotation supporting device that is characterized by using a very small amount of non-grease oil as a lubricant.
However, the ball bearings that have the cage impregnated with a lubricating oil as taught in the Unexamined Japanese Patent Publications Nos. Sho. 61-6429 and Hei. 1-93623 suffer from the disadvantage of high impregnation and, hence, there is the need to make the cage from either a porous material or a special one that will readily absorb oils, causing an inevitable increase in the manufacturing cost. The rotation supporting device that is taught in the Unexamined Japanese Patent Publication No. Sho 64-46011 is not always capable of assuring the necessary amount of lubricating oil and, depending on the use condition, it would be difficult to guarantee satisfactory endurance.